Optical filters

ABSTRACT

An optical filter includes a volume diffraction grating provided within a waveguide. The grating has a thickness T sufficient such that when an optical beam R is incident on the grating from outside the waveguide, wavelengths at or near the Bragg wavelength for the grating are diffracted and coupled into the waveguide, all but the first order interferences being substantially eliminated, while wavelengths away from the Bragg wavelength pass through the waveguide substantially undiffracted. The grating pitch L may be varied to permit tuning of the filter response.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to optical filters.

2. Related Prior Art

Volume reflection grating filters have been proposed for use inwavelength multiplexed optical systems. An example of such a filter isdescribed in published U.K. Patent Application GB No. 2151036. However,such reflection filters are difficult to implement in practice since anincident light beam is reflected back towards the launch directionmaking it difficult to couple the filtered wavelength into a detector orother output device. Furthermore, with reflection filters, once adesired wavelength has been selected by the grating, the filteredwavelength must generally be separately focussed into an output opticalfiber for onward transmission.

SUMMARY OF THE INVENTION

In accordance with the present invention, an optical filter comprises anoptical waveguide within which is provided a volume diffraction gratingof refractive index modulations, the grating having a thicknesssufficient such that when an optical beam is incident on the gratingfrom outside the waveguide, wavelengths in a predetermined range at ornear the Bragg wavelength for the grating are diffracted and coupledinto the waveguide, all but the first order diffractions beingsubstantially eliminated, whilst wavelengths away from the Braggwavelength pass through the waveguide substantially undiffracted.

The filter according to the invention makes use of the wavelengthselectivity of a volume diffraction grating in which light at onewavelength can be efficiently diffracted whilst light of a secondwavelength passes through the diffraction grating with no effect. Thethickness of the grating enables diffraction of harmonic wavelengthcomponents to be eliminated (i.e. higher than first order diffractionsare suppressed). Preferably Q is greater than about 10 where Q=2πλt/nd²,d being the fringe spacing. Moreover, by appropriately varying thegrating thickness, the bandwidth of the range of wavelengths at or nearthe Bragg wavelength which are efficiently diffracted by the grating ata given angle of incidence of the input beam may be convenientlyadjusted as desired. The present filter is further distinguished fromthe conventional reflection gratings of GB No. 2151036 since the gratingis provided in a waveguide. This enables a relatively thick (in thedirection of grating normal) grating to be constructed providing theadvantage of narrower bandwidth operation. In addition the directcoupling between the grating and waveguide results in a much simpleroverall construction.

In order to achieve good coupling into the waveguide the gratingmodulations are conveniently established in planes extending at least inone direction normal to the preferred axis of propagation in thewaveguide and extending in another direction, orthogonal to the onedirection, at an angle π/2-φ with respect to the axis of propagation.

For coupling from an external optical beam into the waveguide thegrating inclination φ should be less than π/2. In operation the opticalbeam should then be incident at an angle π/2-θ to the axis ofpropagation, where θ<φ and θ=2φ-π/2. For a given grating thickness, thecoupled bandwidth is narrower the closer θ approaches to π/2. Fornarrowband operation, it is therefore generally preferable for φ (andthus θ) to be as close to π/2 as possible.

However, as φ approaches π/2, the required inclination θ of the inputbeam also approaches π/2. In these circumstances, as θ increases, in theabsence of efficient, index-matched coupling between the input beam andthe waveguide, an increasing proportion of the input beam is simplyreflected away from the waveguide before interaction with the gratingleading to a corresponding reduction in the amount of light availablefor coupling into the waveguide. Practically, therefore, it is necessaryto find a compromise. Without taking special steps towards indexmatching, it has been found that a satisfactory balance betweennarrowness of bandwidth and the proportion of coupled power isobtainable for angle φ of approximately 5π/12 (θ˜π/3).

Preferably, the grating pitch varies along the waveguide, the wavelengthband which is diffracted depending on the grating pitch at the positionof incidence of the optical beam.

Preferably, the filter further comprises light guide means for guidingthe light beam onto the grating, and tuning means to effect a relativemovement of the light guide means and the grating, whereby the filtercan be tuned to diffract a predetermined wavelength by suitablypositioning the light guide means and the grating relative to oneanother.

The volume nature of the grating and variation in the grating pitchallow a tunable filter to be conveniently implemented. Since the pitchvaries along the waveguide a beam diffracted by one section of thegrating will not be affected by other parts of the grating.

Furthermore, the filtered beam will exit from the grating along thewaveguide and thus will not interfere with the input optical beam or theinput beam optics. Similarly, the input beam will not interfere with theoutput beam or output beam optics leading to considerable ease of use.

The filter has particular advantage for use as a channel selection or achannel dropping filter in wavelength multiplexed (WDM) fibre opticnetworks.

The filter can be incorporated in an optical device including anauxiliary waveguide onto which the filter is mounted, the arrangementbeing such that a beam diffracted by the filter grating in use iscoupled into the auxiliary waveguide.

In this specification, the term optical is intended to refer to thatpart of the electro-magnetic spectrum which is generally known as thevisible region together with those parts of the infrared and ultravioletregions at each end of the visible region which are capable of beingtransmitted by dielectric optical waveguides such as optical fibres.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of an optical filter in accordance with the presentinvention and methods of operation will now be described with referenceto the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an embodiment of a filter according tothe invention;

FIG. 2 illustrates schematically an unslanted volume diffractiongrating;

FIGS. 3 and 4 are graphs illustrating the wavelength and angularsensitivity of filters according to the invention;

FIG. 5 is a graph showing the grating thickness required for 100%diffraction efficiency;

FIG. 6 is a graph of the spectral response of a filter according to theinvention;

FIG. 7 illustrates an optical device including the filter of FIG. 1; and

FIGS. 8 and 9 are schematic diagrams showing how a volume diffractiongrating may be written in a waveguide.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The filter shown in FIG. 1 comprises a film 1 defining a volume (phase)diffraction grating having a series of slanted diffraction planes 2, thegrating normal 8 to the diffraction planes 2 extending in the zx planeof the film. In operation, an incident optical beam is transmitted alongan optical fibre 3 via a collimating lens 4 which directs the beam ontoa first surface 1A of the film 1 at an angle θ to the film normal 5. Thegrating normal 8 is at an angle φ to the film normal 5. In this casecomponents of the incident light beam at the relevant wavelength(s) arediffracted with the first order diffracted beam S being transmitted atright angles to the film normal 5. As shown in FIG. 1, the diffractedbeam S then passes via an end surface 1B of the grating waveguide 1 toan optical detector 6. Alternatively, for example, the diffracted beamcould be coupled directly into another optical waveguide. The remainingwavelengths pass undiffracted straight through the grating 1.

As illustrated, the grating is of fixed pitch and consequently thewavelength response for a given incident angle θ is substantially thesame wherever the input beam R is incident on the grating. It ispossible to vary the pitch along the length of the grating to provide aso-called chirped grating. In such a case the grating pitch is madelocally periodic over distances of the order of the width of the inputbeam but changes over longer distances. This enables the response of thefilter to be turned simply by translanting the input optical fibre 3 andconverging lens 4 lengthwise along the grating 1 so that the light beamR is incident on a section of the grating 1 with a pitch appropriate forthe desired filtering response.

To explain the device response it is helpful to consider a simplifiedmathematical analysis as presented below.

The device shown in FIG. 1 is not immediately amenable to analysis byusing Kogelnik's coupled wave theory ("Coupled wave theory for thickhologram gratings" H. Kogelnik, Bell System Technical Journal, vol 48,No. 9, pp 2909 Nov. 1969.) This theory requires the S wave to propagatein the z direction, which allows a solution to the wave equations to befound at the boundary z=T, after propagation through the thickness T ofthe grating (see FIG. 1). However, this is not a fundamental physicallimitation and rotation of the axes allows a solution to be obtainedwhich is applicable to the present filter. For convenience, therefore,the geometry that will be analysed here is that of the unslanted grating7 shown in FIG. 2 (where φ=0). The diffraction efficiency η for thisarrangement is given by ##EQU1##

Δλ and Δθ represent the deviations in λ and θ from the Bragg condition.From these results, it is apparent that appropriate values of gratingpitch L, base refractive index n_(o) and index modulation n₁ may beselected such that the grating has 100% diffraction efficiency at agiven wavelength λ, input at angle θ.

For a fixed set of grating parameters, it is possible to determine theangular and wavelength sensitivity of the filter structure. FIG. 3 showsa series of response curves for different index modulations n₁ and fordifferent incident angles θ illustrating how the bandwidth is narrowedas θ is increased and as the index modulation n₁ is decreased. FIG. 4shows a similar series of curves illustrating a corresponding change inangular bandwidth Δθ. FIG. 5 shows the grating thickness required toachieve 100% diffraction efficiency under the given conditions.

For a given index modulation n₁, the operational bandwidth is narroweras θ approaches π/2. However, as θ increases, particularly above aboutπ/3, reflection losses at the boundary interface with the waveguide riseproportionately more rapidly. Therefore, unless special measures aretaken to reduce such losses, for example by using suitable indexmatching techniques, there is little to be gained by increasing θ aboveπ/3 (as can be seen from FIGS. 3 to 5).

From a straightforward geometrical analysis it can be seen that forlight to be coupled into the waveguide then the grating slant angle φmust be greater than the angle of beam incidence θ and the two anglesmust be related such that θ=2φ-π/2. It will also be seen that therequired grating thickness is reduced as φ-θ→0.

Given the above restrictions, for φ=5π/12 and θ=π/3, FIG. 6 illustratesthe spectral response of a filter according to the invention with indexmodulation n₁ of 2×10⁻² and a grating length of 200 μm with a centrewavelength of 1.3 μm. The response is of the form sin(x)/x with a seriesof decreasing maxima. The relative amplitude of these peaks may beadjusted by varying the index modulation. (The analysis after Kogelnikassumes a sinusoidal variation.)

The volume diffraction grating may be provided in a waveguide whichcomprises a suitable holographic medium, such as for example dichromatedgelatin (DCG). Waveguides comprising optically non-linear materialshaving stable non-linear states (i.e. non-linearity decay timesrelatively longer than the duration of an optical input requiringfiltering) may also be used. Waveguides of the kind described incopending patent application GB No. 8722014 in the name of the presentapplicants may be suitable.

FIG. 8 illustrates schematically how the slanted volume diffractiongrating for a fixed wavelength filter may be written in a waveguide. Inthis case, the waveguide 80 is placed at an appropriate angle in thezone of interference 83 between two collimated laser beams 81,82. FIG. 9shows one method by which a chirped volume diffraction grating for atunable filter may be produced. As before, a waveguide 90 with suitableholographic properties is positioned similarly in the interference zone93 between two laser beams 91,92. However, in this instance the beamsare made to diverge to create the desired chirping in the fringes.

Filters according to the invention may conveniently be used with otheroptical components. FIG. 7 illustrates an optical device incorporatingthe filter of FIG. 1 together with an auxiliary waveguide comprising aconventional optical fibre. The filter (referenced 9) is incorporatedinto a polished coupler 10 with a conventional optical fibre 11. When abeam R is incident on the filter, the diffracted, filtered beam S isthen coupled into the optical fibre 11 as indicated by the arrowed path.

We claim:
 1. An optical filter comprising:an optical waveguide withinwhich is provided a volume diffraction grating of refractive indexmodulations, the grating having a thickness sufficient such that when anoptical beam is incident on the grating from outside the waveguide,wavelengths at or near a predetermined Bragg wavelength for the gratingare diffracted and coupled into the waveguide, all but the first orderdiffraction being substantially eliminated, while wavelengths away fromthe Bragg wavelength pass through the waveguide substantiallyundiffracted.
 2. An optical filter according to claim 1, wherein thegrating modulations are established in planes extending at least in onedirection substantially normal to the preferred axis of propagation inthe waveguide and extending in another direction, orthogonal to the onedirection, at an angle π/2-φ with respect to the axis of propagation,where φ is less than π/2.
 3. An optical filter according to claim 2,wherein φ is not more than 5π/12.
 4. An optical filter according toclaim 1 wherein the diffraction grating has a pitch which varies alongthe waveguide.
 5. An optical filter according to claim 1 furthercomprising light guide means for guiding a light beam onto the grating,and tuning means to effect a relative movement of the light guide meansand grating, whereby the filter can be tuned to diffract a predeterminedwavelength by suitably positioning the light guide means and gratingrelative the one another.
 6. An optical filter according to claim 1wherein the waveguide comprises an optical fibre having a corecontaining an optically non-linear medium.
 7. An optical devicecomprising a filter according to claim 1, and an auxiliary waveguideonto which the filter is mounted, the arrangement being such that thebeam diffracted by the filter grating in use is coupled into theauxiliary waveguide.
 8. Apparatus for selectively diffracting opticalsignals of predetermined wavelength, said apparatus comprising:a volumediffraction grating disposed within an optical waveguide having alongitudinal axis along which said diffraction grating is also disposed;and optical input means for directing input optical signals includingsignals over a range of different wavelengths, into said waveguide at anacute, non-zero, angle with respect to a normal to said longitudinalaxis; said diffraction grating having a sufficient thickness dimensiontransverse to said longitudinal axis to cause input optical signals ofthe said predetermined wavelength to be diffracted and to pass alongsaid longitudinal axis within the waveguide while other input opticalsignals having other wavelengths are not so diffracted.
 9. Method forselectively diffracting optical signals of predetermined wavelength,said method comprising:disposing a volume diffraction grating within anoptical waveguide having a longitudinal axis along which saiddiffraction grating is also disposed; and directing optical inputsignals including signals over a range of different wavelengths, intosaid waveguide at an acute, non-zero, angle with respect to a normal tosaid longitudinal axis; providing a sufficient thickness of saiddiffraction grating transverse to said longitudinal axis to cause inputoptical signals of said predetermined wavelength to be diffracted and topass along said longitudinal axis within the waveguide while other inputoptical signals having other wavelengths are not so diffracted.